System and method for time reversal situational awareness

ABSTRACT

A system and method are disclosed for providing time reversal situational awareness. An input filter receives an input signal at a first frequency and a pump signal at a second frequency and blocks signals at a third predetermined frequency. A non-linear medium performs a second order operation or a third order operation on the filtered input signal and the filtered pump signal to produce an output signal at the third frequency, the third frequency lower than the first frequency and the second frequency. An output filter is coupled to receive the output signal from the non-linear medium and blocks signals at the first frequency and at the second frequency such that a filtered output signal is generated that is a phase conjugate signal which provides time reversal situational awareness.

FIELD

This disclosure relates generally to a system and method for time reversal situational awareness.

BACKGROUND

Situational awareness is commonly defined as the perception of environmental elements with respect to time or space, the comprehension of their meaning, and the projection of their status after some variable has changed, such as time, or some other variable, such as a predetermined event. Situational awareness is understanding of the environment critical to decision-makers in areas involving complex and dynamic tasks (e.g., aviation, air traffic control, ship navigation, power plant operations, military command and control, and emergency services) to more ordinary but still complex tasks (e.g., automobile operation). Situational awareness is improved based on the use of sensors to obtain status information about the particular environmental elements. However, even with a full set of sensors monitoring the environmental elements of interest, there may be points in time when events relating to such environmental elements have occurred that were missed. It would thus be desirable to have a way to reconstruct in either physical or informational space, situations that no longer exist to identify events not captured by the set of sensors.

Accordingly, there is a need for a system and method for time reversal situational awareness that is performed independently from acquired sensor data.

SUMMARY

In a first aspect, a system for providing time reversal situational awareness based on a received input signal at a first predetermined frequency includes an input filter, a non-linear medium, and an output filter. The input filter receives the input signal and a pump signal at a second predetermined frequency. The input filter is configured to block signals at a third predetermined frequency. The non-linear medium is coupled to an output of the input filter for receiving the filtered input signal and the filtered pump signal. The non-linear medium is configured to perform a second order operation or a third order operation on the filtered input signal and the filtered pump signal to produce an output signal at the third predetermined frequency. The third predetermined frequency is lower than the first predetermined frequency and the second predetermined frequency. The output filter is coupled to receive the output signal from the non-linear medium. The output filter is configured to block signals at the first predetermined frequency and at the second predetermined frequency such that a filtered output signal is generated that is a phase conjugate signal which provides time reversal situational awareness.

In a second aspect, a method for providing time reversal situational awareness includes three steps. First, an input signal at a first predetermined frequency and a pump signal at a second predetermined frequency are filtered to block signals at a third predetermined frequency. Second, a second order operation or a third order operation is performed on the filtered input signal and the filtered pump signal to produce an output signal at the third predetermined frequency. Third, the output signal from the non-linear medium is filtered to block signals at the first predetermined frequency and at the second predetermined frequency such that a filtered output signal is generated that is a phase conjugate signal which provides time reversal situational awareness.

The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and not intended to limit the present disclosure solely thereto, will best be understood in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of a system for providing a time-reversal situational awareness output signal in accordance with a first embodiment;

FIG. 2 is a block diagram of a system for providing a time-reversal situational awareness output signal in accordance with a second embodiment;

FIG. 3 is a block diagram of an input filter for use in the system for providing a time-reversal situational awareness output signal of the present disclosure;

FIG. 4 is a block diagram of an input filter for use in the system for providing a time-reversal situational awareness output signal of the present disclosure;

FIG. 5 is a block diagram of a system for providing a time-reversal situational awareness output signal in accordance with a third embodiment; and

FIG. 6 is a block diagram of a system for providing a time-reversal situational awareness output signal in accordance with a fourth embodiment.

DETAILED DESCRIPTION

In the present disclosure, like reference numbers refer to like elements throughout the drawings, which illustrate various exemplary embodiments of the present disclosure.

It is well known that certain types of nonlinear optical processes involve the generation of a conjugate wave. The most common is the so-called phase conjugation process. In this four-wave mixing process, laser signals interact in such a way as to generate a signal whose propagation is the algebraic conjugate of the input. In a mathematical concept, a wave of exp(k·x+iωt) becomes exp(k·x−iωt). As such, the wave appears to “run backwards” although the signal is not really going backwards in time. Instead, the wave is returning to a prior condition. However, this is not only the case for straight phase conjugation. A phase conjugate signal (i.e., a signal where the term “+iωt” is changed to “−iωt”) occurs any time that a nonlinear optical process involves the creation of a wave that has a lower frequency that the input wave. For example, in stimulated Brillouin scattering, the lower frequency output signal travels with a k·x−iωt propagation, even though the ω will have shifted in value from the original.

Applying this to an imaging situation, if a coherent signal (e.g., an input signal generated as a result of active illumination) is received and put through a conjugating process, the output signal will include the −iωt term and thus seem to move backwards in time. In other words, the output signal will show the state of the image before the input signal was actually received. This results from the physical response to a nonlinear medium to reverse the propagation of the signals, leading to knowledge of the system before the coherent images were received. In order to create a light beam with the time-reversed propagation, it is important to generate a beam that is shifted to a lower frequency, via a nonlinear optical process. These non-linear optical processes fall broadly into two categories, second order and third order processes.

In a first version of a second order case shown in FIG. 1, a system 100 for applying difference frequency generation (DFG) is shown which includes a common input filter for both input sources. Difference frequency generation (DFG) is a second order operation. An image at a frequency ω_(I) 120 is a first input (i.e., an input signal) and a pump laser frequency ω_(P) 110 is a second input (i.e., a pump signal). The two inputs 110, 120 are first applied to an input optical filter 130 and then the filtered versions are applied to a non-linear medium χ⁽²⁾ 140, and then to an output filter 150. Examples of a non-linear medium that could be used for non-linear medium χ⁽²⁾ 140 include (but are not limited to): Potassium Titanyl Phosphate (KTP), Potassium DiHydrogen Phosphate (KDP), Potassium Titanyl Arsenate (KTA), Lithium Triborate (LBO), Beta Barium Borate (BBO), and Lithium Niobate (LiNbO3). By performing DFG with a pump laser frequency ω_(P), a mixing signal ω_(DFG) 160 is produced, where ω_(DFG)=ω_(P)−ω_(I). Given that these conditions are satisfied, a signal k_(DFG) is then a conjugate signal.

The input optical filter 130 blocks any stray light at the frequency ω_(DFG), the output optical filter 150 transmits light at frequency ω_(DFG), while blocking light at the frequencies ω_(I) and ω_(P). This ensures that the output signal ω_(DFG) 160 results only from the nonlinear interaction of the nonlinear medium χ⁽²⁾ 140. Since the propagation function is a conjugate to the signal, allowing the signal to propagate will thus recreate ω_(I) at any earlier point in time, with only a frequency shift.

In a second version of a second order case shown in FIG. 2, a system 101 for applying difference frequency generation (DFG) is shown which includes separate input filters for each input source. An image at a frequency ω_(I) 120 is a first input and a pump laser frequency ω_(P) 110 is a second input. The two inputs 110, 120 are each applied to a respective separate input optical filter 135, 136 and then the filtered versions are applied to a non-linear medium χ⁽²⁾ 140, and then to an output filter 150. By performing DFG with a pump laser frequency ω_(P), a mixing signal ω_(DFG) 160 is produced, where ω_(DFG)=ω_(P)−ω_(I). Given that these conditions are satisfied, a signal k_(DFG) is then a conjugate signal.

In the FIG. 2 embodiment, each input optical filter 135, 136 blocks any stray light at the frequency ω_(DFG). In addition, input optical filter 135 also blocks light at the frequency ω_(I) while input optical filter 136 also blocks light at the frequency ω_(P). The output optical filter 150 transmits light at frequency ω_(DFG), while blocking light at the frequencies ω_(I) and ω_(P). This ensures that the output signal ω_(DFG) 160 results only from the nonlinear interaction of the nonlinear medium χ⁽²⁾ 140. Since the propagation function is a conjugate to the signal, allowing the signal to propagate will thus recreate ω_(I) at any earlier point in time, with only a frequency shift.

Referring now to FIG. 3, input optical filter 130 includes an intensity control element 131, a polarization control element 132 and a frequency control element 133. The frequency control element 133 must be the final element in input filter 130, although the order of intensity control element 131 and polarization control element 132 may be reversed. Intensity control element 131 is a lens or lens combination designed to control the diameter of the light beam that propagates through the nonlinear material 140 in FIG. 1. Polarization control element 132 is a combination of polarizer and waveplate used to control the polarization of the light beam that propagates through the nonlinear material 140 in FIG. 1. Frequency control element 133 is an optical filter designed to transmit light at frequencies ω_(I) and ω_(P), while not transmitting any light at frequency ω_(DFG). Input optical filters 135, 136 of FIG. 2 include the same elements of an intensity control element, a polarization control element and a frequency control element, with the only difference being that the respective frequency control element in input optical filters 135, 136 is configured to only pass light at the frequency of light of the associated input source (i.e., ω_(I) or ω_(P)) and to block light at the frequency of light of the other input source (i.e., ω_(P) or ω_(I)).

Referring now to FIG. 4, the output optical filter 150 includes a frequency control element 151, a polarization control element 152 and an intensity control element 153. The frequency control element 151 is an optical filter designed to block light at frequencies ω_(I) and ω_(P), while transmitting light at frequency ω_(DFG). The frequency control element 151 is the first element in output optical filter 150, while the order of polarization control element 152 and intensity control element 153 may be reversed. Polarization control element 152 is a combination of polarizer and waveplate used to control the polarization of the light beam that exits output optical filter 150. Intensity control element 153 is a lens or lens combination designed to control the diameter of the light beam that exits output optical filter 150.

A first version of a third order case is shown in FIG. 5 which is analogous to the second order case of FIG. 1, substituting a third-order nonlinear medium χ⁽³⁾ 240 for the second-order nonlinear medium χ⁽²⁾ 140 of FIG. 1 to produce an output signal ω_(SRS) 260. Examples of a non-linear medium that could be used for non-linear medium χ⁽³⁾ 240 include (but are not limited to): Potassium Titanyl Phosphate (KTP), Potassium DiHydrogen Phosphate (KDP), Potassium Titanyl Arsenate (KTA), Lithium Triborate (LBO), Beta Barium Borate (BBO), and Lithium Niobate (LiNbO3). The third-order nonlinear medium χ⁽³⁾ 240 may involve stimulated Raman scattering, stimulated Brillouin scattering, or phase conjugation (all third order operations), with the constraint that the output signal must be at a lower frequency than the input signal. In particular, for stimulated Raman scattering (SRS), the output signal ω_(SRS) 260 equals 2*(ω_(I)−ω_(P)), subject to the condition that ω_(P)>ω_(I). As with the FIG. 1 embodiment, the input filter 130 passes ω_(I) and ω_(P) but blocks any spurious signal at the frequency of the output signal ω_(SRS). The output filter 150 only passes signals at ω_(SRS), blocking light at the frequencies ω_(I) and ω_(P). Given that these conditions are satisfied, a signal k_(SRS) is then a conjugate signal.

A second version of a third order case shown in FIG. 5 which is analogous to the second order case of FIG. 2, substituting a third-order nonlinear medium χ⁽³⁾ 240 for the second-order nonlinear medium χ⁽²⁾ 140 of FIG. 2 to produce an output signal ω_(SRS) 260. The third-order nonlinear medium χ⁽³⁾ 240 may involve stimulated Raman scattering, stimulated Brillouin scattering, or phase conjugation, with the constraint that the output signal must be at a lower frequency than the input signal. In particular, for stimulated Raman scattering (SRS), the output signal ω_(SRS) 260 equals 2*(ω_(I)−ω_(P)), subject to the condition that ω_(P)>ω_(I). As with the FIG. 1 embodiment, the input filter 130 passes ω_(I) and ω_(P) but blocks any spurious signal at the frequency of the output signal ω_(SRS). The output filter 150 only passes signals at ω_(SRS), blocking light at the frequencies ω_(I) and ω_(P). Given that these conditions are satisfied, a signal k_(SRS) is then a conjugate signal.

The effective nonlinear polarization can be written as:

{right arrow over (P)} _(S) ⁽³⁾({right arrow over (k)} _(s) =−{right arrow over (k)} _(i),ω)=χ⁽³⁾(ω):E ₁(k ₁)E′ ₁(−k ₁)E _(i)*(k _(i))  (1)

where E₁, E′₁ and E_(i)* are incident waves. (The subscript “s” on the output polarization represents the output wave.) It should be noted that each incident waves has an independent K value. It should also be noted that the K vector for the third order polarization is equal to the negative incident, −k_(i). From symmetry considerations the third order polarization can then be expressed as:

=A(E ₁ ·E _(i)*)E ₁ ′+B(E ₁ ′·E _(i)*)E ₁ +C(E ₁ ·E ₁′)E _(i)*  (2)

where A, B and C are coefficients that all depend on an angle θ between the E₁ and E_(i), and B(θ)=A(π−θ). Since these are all vector quantities, by properly arranging the polarizations of the three incoming waves, it is possible to have only one term nonzero. If this nonzero term is the third term, then the amplitude is E₁*, the phase conjugate of one of the inputs. This signal, derived from the third order polarization, is the one that is used to reconstruct the original configurations since the conjugate wave will have a propagation term that is effectively running in reverse from the input signal (as discussed above).

There are a number of different applications for the system and method for time reversal situational awareness disclosed herein. In imaging, for example, if a signal is being monitored that provides an image of an object, that signal provides a time dependent representation of what the object is doing from the point at which data is being collected. However, no information is available relative to what the object was doing before the data collection began. The phase conjugate signal discussed above provides the ability to back calculate what the object was doing for a period of time before the data collection began. This can be useful for inspection purposes, etc. Note that the behavior that can be identified depends on the collected signal and whether there may have been a major “state” change in the object (e.g., movement of the object into the path of the imaging device). But for some span of time (e.g., back to the time in which the object was placed into the path of the imaging device), information about the behavior of the object may be obtained.

The phase conjugate signal is also useful for image reconstruction. As light travels through a dispersive medium such as air in the atmosphere, the propagation of the light can be disrupted. By obtaining the phase conjugate signal for the received light signal, information can be obtained about what the image looked like as the light signal left the object prior to the dispersal of that light signal in the air (i.e., the image will not include noise effects caused by the dispersive medium). Similarly, the phase conjugate signal is useful for ranging reconstruction. The same notion applies to a LIDAR signal (a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light), an optical ranging signal can be reconstructed to identify the distortions caused by the air in the atmosphere. Finally, the phase conjugate signal can be used for communications signal reconstruction. Any communications signal is subject to “breakup” when traveling through a medium such as the air or a fiber optic pathway. The phase conjugate signal can provide information about the signal source (e.g., physical position, etc.) prior to the initiation of data collection.

Although the present disclosure has been particularly shown and described with reference to the preferred embodiments and various aspects thereof, it will be appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the disclosure. It is intended that the appended claims be interpreted as including the embodiments described herein, the alternatives mentioned above, and all equivalents thereto. 

1. A system for providing time reversal situational awareness, comprising: an input filter for receiving an input signal at a first predetermined frequency and a pump signal at a second predetermined frequency, the input filter configured to block signals at a third predetermined frequency; a non-linear medium coupled to an output of the input filter for receiving the filtered input signal and the filtered pump signal, the non-linear medium configured to perform a second order operation or a third order operation on the filtered input signal and the filtered pump signal to produce an output signal at the third predetermined frequency, the third predetermined frequency lower than the first predetermined frequency and the second predetermined frequency; and an output filter coupled to receive the output signal from the non-linear medium, the output filter configured to block signals at the first predetermined frequency and at the second predetermined frequency such that a filtered output signal is generated that is a phase conjugate signal which provides time reversal situational awareness.
 2. The system of claim 1, wherein the non-linear medium performs a second order operation.
 3. The system of claim 2, where the second order operation is a difference frequency generation operation.
 4. The system of claim 1, wherein the non-linear medium performs a third order operation.
 5. The system of claim 4, wherein the third order operation is a stimulated Raman scattering operation.
 6. The system of claim 4, wherein the third order operation is a stimulated Brillouin scattering operation.
 7. The system of claim 4, wherein the third order operation is a phase conjugation operation.
 8. The system of claim 1, wherein the input signal comprises an image of an object, and wherein the filtered output signal is used to identify information about the object prior to receipt of the input signal.
 9. The system of claim 1, wherein the input signal comprises a light signal of an image of an object, wherein the light signal has passed through a dispersive medium, and wherein the filtered output signal recreates the image of the object without any noise caused by the dispersive medium.
 10. The system of claim 1, wherein the input signal comprising an optical ranging signal that has passed through a dispersive medium, and wherein the filtered output signal is a reconstructed version of the optical ranging signal that removes distortions caused by the dispersive medium.
 11. A method for providing time reversal situational awareness, comprising the steps of: filtering a received input signal at a first predetermined frequency and a pump signal at a second predetermined frequency to block signals at a third predetermined frequency; performing a second order operation or a third order operation with a non-linear medium on the filtered input signal and the filtered pump signal to produce an output signal at the third predetermined frequency; and filtering the output signal from the non-linear medium to block signals at the first predetermined frequency and at the second predetermined frequency such that a filtered output signal is generated that is a phase conjugate signal which provides time reversal situational awareness.
 12. The method of claim 11, wherein the non-linear medium performs a second order operation.
 13. The method of claim 12, where the second order operation is a difference frequency generation operation.
 14. The method of claim 11, wherein the non-linear medium performs a third order operation.
 15. The method of claim 14, wherein the third order operation is a stimulated Raman scattering operation.
 16. The method of claim 14, wherein the third order operation is a stimulated Brillouin scattering operation.
 17. The method of claim 14, wherein the third order operation is a phase conjugation operation.
 18. The method of claim 11, wherein the input signal comprises an image of an object, and wherein the filtered output signal is used to identify information about the object prior to receipt of the input signal.
 19. The method of claim 11, wherein the input signal comprises a light signal of an image of an object, wherein the light signal has passed through a dispersive medium, and wherein the filtered output signal recreates the image of the object without any noise caused by the dispersive medium.
 20. The method of claim 11, wherein the input signal comprising an optical ranging signal that has passed through a dispersive medium, and wherein the filtered output signal is a reconstructed version of the optical ranging signal that removes distortions caused by the dispersive medium. 